CdZnTe(110)面表面原子弛豫和表面态的研究

通过真空Ar+离子刻蚀获得理想清洁的(110)表面,采用低能电子衍射(LEED)观察了(110)面表面原子结构,观察到(110)面表面未发生重构.采用光电子能谱技术验证了(110)面表面原子结构发生了弛豫.采用角分辨光电子谱(angle-resolved photoemission spectropy)实验测量出在费米能级以下0.9eV处存在峰宽约为0.8eV的表面态.并估算出其表面电荷密度约为6.9×1014cm-2,亦即表面Te和Cd(Zn)原子各有一个悬键.     

CdTe和CdZnTe电子结构及成键机理的第一性原理研究

采用基于密度泛函理论的CASTEP程序软件包,计算了CdTe和CdZnTe的能带结构和态密度,研究对比了CdTe 和CdZnTe的分子内成键机制.结果表明,在CdZnTe中,Cd eg和Te 5s杂化形成较低的价带,Zn t2g和Te 5pz杂化形成的π键等构成中间价带,Zn 4s和Te 5pz杂化形成的σ键等构成导带.Cd 5s和Te 5s不再杂化,导带底向高能级方向移动.     

基于第一性原理的Au、Cu、Sb掺杂CdTe的结构模拟和光学性能预测

基于密度泛函理论研究了Au、Cu、Sb掺杂CdTe体系的电子结构和光学性能。Au、Cu、Sb掺杂CdTe体系均能稳定存在，过渡金属原子与Cd原子轨道的杂化减小了CdTe的带隙，提高了CdTe对可见光的利用，同时降低了光生电子从价带跃迁到导带所需的能量，从而促进了更多的光生电子发生迁移，大大提高了其光学性能。三种掺杂体系中Sb/CdTe体系在可见光范围内光吸收系数提升最显著，其光生电子和光生空穴迁移率相对于CdTe体系分别增加5.97倍和15.54倍。通过计算掺杂体系的能带、态密度、电子布居、光吸收函数、载流子迁移率，从理论上揭示了Au、Cu、Sb提高CdTe光学性能的机理。     

六方Fe2Ge合金块体及其(001)表面电子结构和磁性的第一性原理

采用基于密度泛函理论第一性原理的赝势平面波方法,计算了块体Fe₂Ge及其(001)表面的电子结构和磁性。考虑了两种类型的终端(001)表面:Ge(Ⅰ)-(001)表面和Ge(Ⅱ)-(001)表面。电子结构方面,不同类型的Fe₂Ge(001)表面都表现出金属特性,这与块体的金属性保持一致。通过计算它们的自旋极化率,得出Ge(Ⅰ)-(001)表面的自旋极化程度最高。磁性方面,在块体和Ge(Ⅱ)-(001)表面的Ge原子是铁磁自旋有序的,而在Ge(Ⅰ)-(001)表面第一层的Ge原子是亚铁磁自旋有序的。此外,Ge(Ⅱ)-(001)表面Ge原子的自旋磁矩优于块体中和Ge(Ⅰ)-(001)表面Ge原子的自旋磁矩。这些结果与Fe的d态和Ge的p态电子的杂化有关,本文中通过分析它们的态密度进行了讨论。     

富Te条件下CdTe晶体中的点缺陷研究

基于缺陷化学理论,考虑到富Te的CdTe晶体中可能存在的点缺陷,建立了在Te气氛下退火时,热力学平衡态晶体中的点缺陷模型,其中包括Cd间隙(Cdi)、Cd空位(VCd)、Te间隙(Tei)和Te反位(TeCd).利用质量作用定律和伪化学平衡方程计算了富Te情况下本征CdTe晶体中的点缺陷浓度和费米能级.计算结果系统的揭示了点缺陷浓度、费米能级、Te压以及退火温度之间的关系,发现只有TeCd浓度足够大时才能对费米能级产生钉扎作用.     

MgH2脱氢动力学的表面缺陷效应研究

采用基于密度泛函理论的第一性原理计算方法,研究了完整、含Mg原子空位及Pd原子掺杂三种MgH2(110)表面的氢脱附行为及其动力学,并从电子结构角度给出了表面空位/掺杂两类缺陷对其脱氢动力学的影响机制.结果显示:MgH2(110)表面六重配位的Mg原子位置是形成Mg空位或Pd掺杂的优先位置;相对于完整表面而言,Mg空位或Pd掺杂均极大地降低了MgH2(110)表面的氢脱附能垒,在一定程度上解释了MgH2纳米结构调制与催化掺杂可明显改善体系脱氢动力学的实验现象;电子结构分析发现,表面空位/掺杂缺陷的存在致使MgH2表面在费米能级附近能隙变窄、低能级区成键电子数减少,进而导致近表面的原子层稳定性降低,从而使得表面Mg-H间相互作用减弱.     

湿氧钝化CdSe(110)表面的XPS分析

用双氧水对CdSe单晶(110)表面进行钝化,采用X射线光电子能谱(XPS)分析了湿氧处理的CdSe(110)表面的化学特征.通过两次不同分析模式FAT(固定通能)和FRR(固定减速比)所得到的结果表明:CdSe表面出现Se偏析,表面上形成了SeOx(x＜1),SeO、Cd(OH)2和CdCO3的高电阻稳定氧化层,消除了器件表面态,可以减少器件的表面漏电流和改善其信噪比.     

Fe对InP材料电子结构及光学性质的调控机理研究

采用基于密度泛函理论的线性缀加平面波(FLAPW)方法,研究了3d族过渡金属元素Fe对Ⅲ-Ⅴ族半导体InP的电子结构和光学性质的调控机理,并对其能带结构和电荷密度分布进行了分析.结果表明,InP为直接带隙半导体,其价带主要由P-3s和3p态构成,而导带则由In-5s电子态构成.当Fe元素替代In原子后,由于Fe和P原子的轨道杂化作用,InP带隙中出现杂质态,Fe-3d态产生自旋极化效应.随着Fe的掺杂浓度增大,Fe-P原子之间轨道杂化作用明显增加,费米能级逐渐进入价带,这导致了材料的电子跃迁几率提高,光学吸收边明显增强,跃迁峰发生红移.     

NbB2（0001）表面性质的第一性原理研究

应用密度泛函理论的第一性原理的方法分析了两种不同终端的NbB2(0001)表面的几何结构和电子结构。结果表明:两种不同终端的(0001)表面结构弛豫主要发生在前三层,并且硼终端的表面弛豫程度小于铌终端表面的弛豫。表面能分析结果表明,终止于硼终端的(0001)表面结构在更宽的范围内具有较低的表面能,即硼终端的(0001)表面比铌终端的表面更稳定。进一步分析NbB2(0001)两种终端表面的电子结构表明:在金属铌和硼之间发生了电子转移,加强了化学键的相互作用,导致第一间层向内弛豫。对于铌终端的表面第一层铌转移到第二层的电子数比硼终端表面第二层铌转移到第一层的电子数多,这是导致铌终端表面弛豫程度大于硼终端表面的主要原因。     

CdZnTe单晶表面原子结构

半导体表面是一个特殊区域,其表面原子结构与体态有很大区别,而且他们对材料的整体性能又起着十分重要的影响.本文通过真空Ar<'+>离子刻蚀并原位退火的办法,获得理想清洁的CdZnTe单晶表面,采用低能电子衍射(LEED)观察了(110)、(111)A面和(111)B面的表面原子结构.发现(110)面为稳定面没有发生重构,而在(111)A面上发现了由空位形成的(31/2×31/2)R 30°重构,在(111)B面上发现了由Te顶戴原子形成的(2×2)重构.     

‘Star‐like’ defects in Cd‐annealed CdZnTe crystals—an experimental study of their origin and formation mechanism

We conducted low‐temperature annealing experiments at temperatures slightly above and below the melting point of Te to clarify the effects of the state of Te inclusions (solid or liquid) upon the formation of ‘star‐like’ defects in Cd‐annealed CdZnTe (CZT). We also carried out post‐growth annealing experiments with and without using Cd vapor to clarify the mechanism of formation of such defects. We demonstrated that these ‘star‐like’ defects are due to the reaction between in‐diffused Cd atoms and the molten Te inclusions, but we found no observable ‘one‐to‐one’ correlation between ‘star‐like’ defects and Te inclusions. The non‐uniform distribution of Te inclusions in the CZT matrix could account for this phenomenon since the punching distance of the dislocations depends on the volume fraction of inclusions within the matrix.     

非金属元素(F,S, Se,Te)掺杂对ZnO/graphene肖特基界面电荷及肖特基调控的理论研究

本文基于第一性原理系统研究了非金属元素F、S、Se、Te掺杂对ZnO/graphene异质结界面相互作用及其电子结构的影响。结果表明,ZnO/graphene异质结层间以范德瓦耳斯力结合形成了稳定的异质结,并且形成了n型肖特基势垒。差分电荷密度图表明graphene层的电子向ZnO层转移,使得graphene层表面带正电,ZnO层表面带负电,在界面处形成了内建电场。当掺入F原子时,异质结呈现欧姆接触;当掺入S、Se、Te原子时,异质结肖特基的接触类型均由n型转变为p型,且有效降低了肖特基势垒的高度,特别是Te原子掺入后,p型肖特基势垒高度降低至0.48 eV,提升了电子的注入效率。本文的研究结果将对相关的场效应晶体管的设计和制造提供一定的参考。     

An XPS study of the early stages of silver photodiffusion in Ag/a-As2S3 films

X-ray induced structural changes at the Ag/As₂S₃ interface are investigated using X-ray photoelectron spectroscopy on the samples prepared within the spectrometer. The as-prepared film consists of stable heteropolar As–S bonds as well as ∼16% S (and As) atoms in lower (higher) electron density configurations such as the –S–S– (–As–As–) segments with ‘wrong’ homopolar bonds. Two distinct stages of the X-ray induced diffusion are revealed. At first, silver reacts with atoms within –S–S– like segments to form Ag–S bonds. In the second stage, the Ag–S bonds decompose due to the reaction of S with As atoms within the –As–As– ‘wrong’ segments to form As–S heteropolar bonds, and silver diffuses away from the interface into the film. The results provide guideline for enhancing silver photodiffusion in chalcogenide glass. The irradiation of the (Ag–Te)/As₂S₃ sample with X-rays shows that not only Ag, but Te also diffuses away from the surface.     

Radial distribution studies of glassy tellurium-silicon alloys

X-ray investigations of glassy tellurium-silicon alloys (Si content 10–40 at%) are described. Alloys with 13 to 27 at% Si were obtained in bulk form, non-crystalline samples with 10, 30, 33, 36 and 40 at% Si were obtained by rapid quenching from the melt by the splat-cooling technique. The interference functions and the radial distribution functions of all alloys show a great similarity. However, the positions of the maxima of the pair distribution function decrease continuously with growing Si content and lead to values which are similar to atomic distances in crystalline Te and Si₂Te₃. The areas of the first two peaks of the RDF, F₁ (rougnly 2) and F₂ (roughly 12), are compared with calculated areas derived from different structural models which are based on the short-range order in crystalline Te with a chain structure and in Si₂Te₃ with a tetrahedron configuration. An interstitial model with Si atoms in holes of Te chains and a random substitution model, where the Te atoms and the Si atoms have the same short-range order, both show a concentration dependence of the areas F₁ contrary to the experimental values. A structural model which assumes the tetrahedron configuration of Si₂Te₃ for Si atoms (coordination number N_Si = 4) and a coordination number N_Te = 2 for Te atoms can describe the short-range order of glassy TeSi alloys in the whole investigated concentration range with 10–40 at% Si.     

CdZnTe晶片表面化学抛光研究

采用不同浓度的Br2-MeOH作为抛光液对CdZnTe进行化学抛光,发现用2％Br2-MeOH腐蚀时速率平稳且易于控制,能有效去除表面划痕,获得光亮表面。AFM分析发现,抛光后表面粗糙度降低30％,平整度增加。XPS分析发现CdZnTe的（111）Cd极性面变成了富Te非极性表面。PL分析发现表面陷阱态密度降低,表面晶格的完整性增强。     

Structure of selenium and tellurium clusters in cavities of NaX zeolite

The structure of NaX zeolite crystals with the Se and Te atoms incorporated into the structure from their vapors has been studied. It is shown that selenium atoms form clusters in the shape of six-and four-membered rings located in the cuboctahedral cavities of the structure framework. Single Se₂ molecules are located in large cavities, whereas Te atoms form four-membered rings located in cuboctahedra. Large cavities are occupied by two alternating configurations of Te atoms forming either a chain consisting of 16 links or an eight-membered ring.     

Influence of partial replacement of Cu by Cd and Zn atoms on polymorphous transformation in Cu<Subscript>1.75</Subscript>Te crystal

A layer Cu_{1.75 ? x}Te single crystal in which some Cu atoms were replaced by Cd and Zn atoms (x = 0.05) was studied by high-temperature X-ray diffractometry. It was established that this replacement substantially affects the number and temperature of polymorphous transformations.     

Phase-field simulations of Te-precipitate morphology and evolution kinetics in Te-rich CdTe crystals

Te precipitates are one of principal defects that form during cooling of melt-grown CdTe or CZT crystals when grown Te-rich. Many factors such as the kinetic properties of intrinsic point defects (vacancy, interstitial, and antisite defects); stresses associated with the lattice mismatch between precipitate and matrix; temperature gradients and extended defects (dislocations, twin and grain boundaries); non-stoichiometric composition; thermal treatment history all affect the formation and growth/dissolution of Te precipitates in CdTe. A good understanding of these effects on Te precipitate evolution kinetics is technically important in order to optimize material processing and obtain high-quality crystals. This research develops a phase-field model capable of investigating the evolution of coherent Te precipitates in a Te-rich CdTe crystal undergoing cooling from the melt. Cd vacancies and Te interstitials are assumed to be the dominant diffusing species in the system, which is in two-phase equilibrium (matrix CdTe and liquid Te inclusion) at high temperatures and three-phase equilibrium (matrix CdTe, Te precipitate, and void) at low temperatures. Using available thermodynamic and kinetic data from experimental phase diagrams and thermodynamic calculations, the effects of Te interstitial and Cd vacancy mobility, cooling rates and stresses on Te precipitate, and void evolution kinetics are investigated.     

Self-selecting vapour growth of bulk crystals – Principles and applicability

A method of self-selecting vapour growth (SSVG) for bulk binary and multernary crystals of semiconducting materials is reviewed comprehensively for the first time. Although it has been developed over three decades, the method is less well known – even though it is physically distinct from the more widely used ‘Piper–Polich’ and ‘Markov–Davydov’ vapour transport bulk growth methods. The means by which growth takes place on a polycrystalline source to form a crystal free from the walls is described. Modelling and empirical observations have been used to establish the characteristics of the almost isothermal temperature fields that drive the transport in SSVG. It is demonstrated that precise control of thermal radiation is a fundamental requirement for tailoring the temperature distribution—a fact that has been used well in the design of horizontal tube furnace growth rigs. Achievements in the growth of useful PbS, PbSe, PbTe, CdTe and ZnTe compound crystals are described. The SSVG method has proved to be particularly well suited to the growth of solid solutions, and the results of growth experiments, and of compositional and structural analysis, are presented for Pb(Se,S), (Pb,Sn)Se, (Pb,Sn)Te, (Pb,Ge)Te, Cd(Te,Se), Cd(Te,S) and (Cd,Zn)Te. The excellent compositional uniformity delivered is attributed to entropy driven mixing in the low thermal gradients present in SSVG.

To date, most SSVG has been done at the <50 g level for research or small scale production use. Prospects for scaling up the growth are considered, there being no barriers identified in principle. However, there is a limitation in that the shape of the grown crystals is not accurately controlled at present. To overcome this, and to offer an alternative method of scaling up, the use of vertical tube systems is explored. A significant additional advantage of the vertical configuration is that it allows for continuous recycling of the source/crystal mass so as to continuously self-refine the increasingly uniform – and crystalline – product. Achievements to date in growing II–VI and IV–VI crystals are described for prototype vertical SSVG systems. Finally, future prospects for the SSVG method in terms of further developments to the method, and the specific materials that will benefit from it are highlighted.